CMOS image sensors are gaining widespread use as digital cameras and digital video cameras become more common. Similar to traditional film, the CMOS image sensor captures an image when exposed to light. The CMOS image sensor typically consists of a large array of pixels that are organized into rows. Normally, the pixels in the array are not all exposed to light at the same time. Rather, the pixels are exposed sequentially, row by row. This method is known as a “rolling shutter”. The exposure time for a single row of pixels is called the exposure period. The total time required to expose and process the pixels in the entire array is known as the frame period.
One problem associated with the rolling shutter method is that the illumination level of the light source may vary over time. This variation is called “flicker”. Light sources exhibiting flicker have peaks in brightness corresponding to each peak of the power line frequency. The resulting light pulses have a flicker period that is half that of the AC power line. When the period of the flicker is longer than the exposure period and shorter than the frame period, the final image has bands of contrasting brightness.
The effects of flicker can be avoided by restricting the exposure period of each pixel row to be an integral multiple of the flicker period. However, the exposure period cannot be reduced below the flicker period. If the illumination is very bright, and the exposure period cannot be reduced, then the image will be overexposed. Overexposure can occur when the scene is illuminated with bright lights, or when a camera is pointed directly at a light source.
FIG. 1 shows a basic three-transistor pixel 101 used in prior art image sensor arrays. A transistor M1 connects the cathode (node 103) of a photodiode 125 to a voltage supply, Vdd 107. The anode of the photodiode 125 is connected to ground. The gate of transistor M1 is connected to a reset signal 109. Transistor M3 connects Vdd 107 to another transistor M5. The gate of transistor M3 is connected to node 103. The gate of transistor M5 is controlled by a row select signal 111, while its source is connected to a column output line 113, from which the output of the pixel 101 is read. Transistor M3 is used as a source follower to buffer the photodiode 125 and prevent it from being loaded down by the column output line 113.
In normal operation, the photodiode 125 is reset to the supply voltage Vdd 107 at the beginning of an exposure period, by asserting the reset signal 109 and charging node 103. As the photodiode 125 is exposed to incident light, it accumulates more charge and the voltage at node 103 decreases. The voltage across the photodiode 125 is indicative of the light intensity that the photodiode 125 has been exposed to over time. At the end of the exposure period, the row select signal 111 is asserted to read out the values of a row of pixels in the image sensor array.
In normal operation, the photodiode 105 is reset to the supply voltage Vdd 107 at the beginning of an exposure period, by asserting the reset signal 109 and charging node 103. As the photodiode 105 is exposed to incident light, it accumulates more charge and the voltage at node 103 decreases. The voltage across the photodiode 105 is indicative of the light intensity that the photodiode 105 has been exposed to over time. At the end of the exposure period, the row select signal 111 is asserted to read out the values of a row of pixels in the image sensor array.
The conversion gain of the pixel 101 is defined as the ratio of the change in voltage to the change in charge of the photodiode 125. The capacitance of the photodiode 125 is determined by calculating the amount of charge stored for a given amount of voltage applied. Therefore, the conversion gain of the three-transistor pixel 101 is inversely proportional to the capacitance at node 103.
Pixel circuits are generally designed to have high conversion gain, to improve the pixel sensitivity under low-light conditions. However, if the lighting conditions are too bright, the photodiode will accumulate too much charge and reach saturation, at which point the voltage at node 103 falls to zero. Further exposure of the photodiode cannot be registered, because the voltage cannot fall below zero. As a result, the output signal of the pixel will be clipped, and the final image will look overexposed.
One solution to the problem of clipping is to reduce the exposure period. However, as previously discussed, reducing the exposure period is problematic in lighting environments with flicker. Therefore, a need remains for a solution for preventing overexposure in an image sensor without reducing the exposure period.